Electromagnetic wave shield film

The present invention relates to an electromagnetic wave shielding film.

In conventional practice, an electromagnetic wave shielding film is attached to a printed wiring board, such as a flexible printed wiring board (FPC), to shield the printed wiring board from external electromagnetic waves.

Usually, an electromagnetic wave shielding film has a configuration in which a conductive adhesive layer, a shielding layer made of a thin metal film or the like, and an insulating layer are sequentially stacked. When such an electromagnetic wave shielding film is thermally pressed while it is overlaid on a printed wiring board, the electromagnetic wave shielding film is adhered to the printed wiring board with an adhesive layer, whereby a shielded printed wiring board is produced. After adhesion, components are mounted onto the printed wiring board by solder reflow. The printed wiring board has a configuration in which printed patterns on a base film are covered with an insulating film.

As an example of such an electromagnetic wave shielding film, Patent Literature 1 discloses an electromagnetic wave shielding film including an insulating resin layer, a shielding layer adjacent to the insulating resin layer, and a conductive adhesive layer provided on the side opposite to the insulating resin layer of the shielding layer, wherein the shielding layer includes a plurality of metal layers having a thickness of 1 μm or less, and a non-metal layer interposed between the plurality of metal layers.

Patent Literature 2 discloses an electromagnetic wave shielding sheet including a conductive layer and an insulating layer, wherein the insulating layer contains a thermosetting resin, a setting agent, and a black coloring agent, and the average primary particle size of the black coloring agent is 20 to 100 nm.

In production of the electromagnetic wave shielding film disclosed in Patent Literature 1, the electromagnetic wave shielding film is transferred between rollers. Static electricity is generated in the insulating layer by contact between the insulating layer and the rollers. In the case where static electricity is generated in the insulating layer, electric current does not flow easily even when the insulating layer is earthed, resulting in accumulation of static electricity.

Subsequently, the electromagnetic wave shielding film is cut, and the cut pieces are stacked and stored.

Accumulation of static electricity in the insulating layer causes problems such as one that the electromagnetic wave shielding film cannot be smoothly cut during cutting, and one that a cut piece of the electromagnetic wave shielding film cannot be easily separated from another cut piece when it needs to be individually taken out for use after storage, because they are attached to each other (i.e., blocking).

In the electromagnetic wave shielding sheet according to Patent Literature 2, the insulating layer contains a black coloring agent having conductivity. However, in the case where static electricity is generated in the insulating layer, even when the insulating layer is earthed, electric current does not flow easily, resulting in accumulation of static electricity. Thus, blocking occurs also in the electromagnetic wave shielding sheet according to Patent Literature 2. In addition, since the average particle size of the black coloring agent is as small as 20 to 100 nm, the insulating layer has smaller irregularities on the surface, which gives gloss to the insulating layer. This causes a problem that when the insulating layer is scratched or the like on the surface, scratch marks or the like are easily visible.

The present invention was made to solve the above problems and aims to provide an electromagnetic wave shielding film from which static electricity that has accumulated during production or transfer can be easily dissipated.

An electromagnetic wave shielding film of the present invention includes a protective layer disposed outermost and a shielding layer stacked inward from the protective layer, wherein the protective layer has a surface resistivity of 1.0×10to 2.0×10Ω/□, and the protective layer contains a binder resin and carbon particles having an average particle size of 0.1 to 15 μm.

In the electromagnetic wave shielding film of the present invention, the protective layer disposed outermost has a surface resistivity of 1.0×10to 2.0×10Ω/□.

When the surface resistivity of the protective layer is in the above range, the protective layer can diffuse static electricity, so that even when static electricity accumulates in the protective layer, the static electricity that has accumulated in the protective layer can be quickly dissipated by earthing.

As a result, static electricity is less likely to accumulate in the entire electromagnetic wave shielding film.

When the surface resistivity is less than 1.0×10Ω/□, static electricity can be dissipated instantly, but dissipation of static electricity tends to cause sparks.

When the surface resistivity is more than 2.0×10Ω/□, even when the protective layer is earthed, electric current does not flow easily. Thus, static electricity is not easily dissipated.

In the electromagnetic wave shielding film of the present invention, the protective layer contains a binder resin and carbon particles having an average particle size of 0.1 to 15 μm.

When the protective layer contains carbon particles having an average particle size in the above range, the carbon particles are moderately exposed on the surface of the protective layer, and the surface resistivity of the protective layer can be set to 1.0×10to 2.0×10Ω/□. In addition, since irregularities are formed on the surface of the protective layer, the gloss of the protective layer can be reduced. Thus, even when the protective layer is scratched or the like, scratch marks or the like are not easily visible.

When the average particle size of the carbon particles is less than 0.1 μm, the carbon particles are buried in the protective layer and are less likely to be exposed on the surface of the protective layer. Thus, even when the protective layer is earthed, electric current does not flow easily, and static electricity is not easily dissipated.

When the average particle size of the carbon particles is more than 15 μm, the irregularities on the surface of the protective layer are too large.

In the electromagnetic wave shielding film of the present invention, the amount of the carbon particles in the protective layer is preferably 5 to 30 wt %.

When the amount of the carbon particles is in the above range, the surface resistivity of the protective layer can be set to 1.0×10to 2.0×10Ω/□. In addition, the protective layer can be suitably made in black, resulting in higher concealability.

The electromagnetic wave shielding film of the present invention may include the protective layer, an insulating layer, a metal layer that functions as the shielding layer, and a conductive adhesive layer that are sequentially stacked.

The electromagnetic wave shielding film of the present invention may include the protective layer, an insulating layer, and a conductive adhesive layer that functions as the shielding layer that are sequentially stacked.

The electromagnetic wave shielding films having one of these features have sufficient electromagnetic wave shielding properties.

Since the protective layer is formed outermost, static electricity that has accumulated can be quickly dissipated.

The present invention can provide an electromagnetic wave shielding film from which static electricity that has accumulated during production or transfer can be easily dissipated.

The electromagnetic wave shielding film of the present invention is specifically described below. However, the present invention is not limited to the following embodiments and can be appropriately modified without changing the gist of the invention.

First, an electromagnetic wave shielding film according to the first embodiment of the present invention is described with reference to the drawing.

is a schematic cross-sectional view of an example of the electromagnetic wave shielding film according to the first embodiment of the present invention.

An electromagnetic wave shielding filmshown inincludes a protective layer, an insulating layer, a metal layer, and a conductive adhesive layerthat are sequentially stacked.

In the electromagnetic wave shielding film, the metal layerfunctions as a shielding layer. In the electromagnetic wave shielding film, the protective layeris disposed outermost, and the metal layeris disposed inward therefrom.

In the electromagnetic wave shielding film, the protective layerhas a surface resistivity of 1.0×10to 2.0×10Ω/□. The surface resistivity of the protective layeris preferably 1.0×10to 1.0×10Ω/□, more preferably 1.0×10to 1.0×10Ω/□.

When the surface resistivity of the protective layeris in the above range, the protective layercan diffuse static electricity, so that even when static electricity accumulates in the protective layer, the static electricity that has accumulated in the protective layercan be quickly dissipated by earthing.

As a result, static electricity is less likely to accumulate in the entire electromagnetic wave shielding film.

When the surface resistivity is less than 1.0×10Ω/□, static electricity can be dissipated instantly, but dissipation of static electricity tends to cause sparks.

When the surface resistivity is more than 2.0×10Ω/□, even when the protective layer is earthed, electric current does not flow easily. Thus, static electricity is not easily dissipated.

In the electromagnetic wave shielding film, the protective layercontains a binder resin and carbon particles having an average particle size of 0.1 to 15 μm. The average particle size of the carbon particles is preferably 1 to 10 μm.

When the protective layercontains carbon particles having an average particle size in the above range, the carbon particles are moderately exposed on the surface of the protective layer, and the surface resistivity of the protective layercan be set to 1.0×10to 2.0×10Ω/□. In addition, since irregularities are formed on the surface of the protective layer, the gloss of the protective layercan be reduced. Thus, even when the protective layeris scratched or the like, scratch marks or the like are not easily visible.

When the average particle size of the carbon particles is less than 0.1 μm, the carbon particles are buried in the protective layer and are less likely to be exposed on the surface of the protective layer. Thus, even when the protective layer is earthed, electric current does not flow easily, and static electricity is not easily dissipated.

When the average particle size of the carbon particles is more than 15 μm, the irregularities on the surface of the protective layer are too large.

The carbon particles may form secondary particles with a size of preferably 1 to 15 μm.

The term “average particle size of the carbon particles” as used herein refers to an average of measurements of major axes of five primary carbon particles that are randomly selected from an image of the surface (vertical×horizontal=250 μm×350 μm) of the protective layertaken with Keyence digital microscope VHX-5000 at a magnification of 1000 times.

In the electromagnetic wave shielding film, the amount of the carbon particles in the protective layeris preferably 5 to 30 wt %, more preferably 10 to 30 wt %.

When the amount of the carbon particles is in the above range, the surface resistivity of the protective layercan be set to 1.0×10to 2.0×10Ω/□.

When the amount of the carbon material in the protective layer is less than 5 wt %, the surface resistivity of the protective layer tends to fall outside of a good range.

When the amount of the carbon material in the protective layer is more than 30 wt %, the irregularities on the surface of the protective layer are too large, which easily results in a lower scratch resistance.

The type of the carbon particles is not limited as long as the surface resistivity of the protective layercan be adjusted. For example, amorphous carbon, graphite, or the like may be used.

In the electromagnetic wave shielding film, the protective layermay contain another carbon material, other than the carbon particles. The surface resistivity of the protective layercan be adjusted with such a carbon material.